tty: fix data race in tty_buffer_flush

tty_buffer_flush frees not acquired buffers.
As the result, for example, read of b->size in tty_buffer_free
can return garbage value which will lead to a huge buffer
hanging in the freelist. This is just the benignest
manifestation of freeing of a not acquired object.
If the object is passed to kfree, heap can be corrupted.

Acquire visibility over the buffer before freeing it.

The data race was found with KernelThreadSanitizer (KTSAN).

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 23de97de09a4d9ca0c9885b2a34949a713ec7f3b..b3a5e33948c192cc44f49de297450d6f928ab8d5 100644 (file)
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -242,7 +242,10 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
        atomic_inc(&buf->priority);
 
        mutex_lock(&buf->lock);
-       while ((next = buf->head->next) != NULL) {
+       /* paired w/ release in __tty_buffer_request_room; ensures there are
+        * no pending memory accesses to the freed buffer
+        */
+       while ((next = smp_load_acquire(&buf->head->next)) != NULL) {
                tty_buffer_free(port, buf->head);
                buf->head = next;
        }